1. Field of the Invention
This invention relates to a method for the adjustment of the temper grade in steel strip for use in production of surface-treated plate, particularly tin plate and tin-free steel plate.
2. Description of the Prior Art
Conventionally, steel strip to be used for the production of surface-treated plate such as, for example, tin plate, is differentiated into various temper grades by adjusting the composition of the starting material steels and the hot rolling conditions to selectively obtain soft and hard cold rolled steel strip and further subjecting the steel strip to either a box annealing process or a continuous annealing process, depending on the temper grade desired to be obtained. This conventional process is not only complex in terms of the steelmaking operation and the hot rolling operation but also suffers from the disadvantage that, particularly in the case of high-grade temper strip, it is exceedingly difficult during the process of adding the alloying elements so that they adjust the C, N, Mn, etc. to fall accurately within their respectively prescribed ranges.
To eliminate this disadvantage, it has been suggested that the differentiation of steel strip into various temper grades might be effected by adjusting the degree of tempering during temper rolling, thus eliminating the need for addition of alloying elements in the steelmaking operation and making it possible to reduce the number of types of steel strip required.
In the adjustment of the temper grade of steel strip by temper rolling, it is possible to use wet temper rolling as a means for obtaining the high reduction rate required for the production of a high-grade temper strip. However, when this wet temper rolling is applied to the production of low-grade temper strip (soft material), there ensue the following two problems:
(1) Compared with dry rolling, there is not much generation of Luders zones during wet temper rolling, and this necessitates a rather high reduction rate in order to avoid yield-point elongation. However, beyond the point at which the phenomenon of yield-point elongation ceases to occur, there is an increase in the temper grade due to work hardening, so that it is difficult to attain the desired low-grade temper. In continuously annealed strip, for example, dry rolling requires a reduction rate of about 1% to eliminate yield-point elongation, whereas wet rolling requires a reduction rate on the order of 3% to obtain the same effect.
(2) In wet temper rolling of steel strip such as soft steel material which inherently involves yield-point elongation, the phenomenon of unstable rolling (jumping) ascribable to the yield-point elongation behavior of the material undergoing the temper rolling occurs at rolling reduction rates of less than about 5%, making it impossible to carry out the rolling at a uniform reduction rate. It is, accordingly, difficult to obtain a low temper-grade steel strip for surface-treated plate by wet rolling at a low reduction rate.